Aluminum heads are the problem. AFR advises that with their heads DO NOT use a torque wrench on any bolt. Just tighten them "snuggly". That applies to the plugs too.
I have a long history with aluminum heads. Use anti-seize compound and triple check that the plugs are threaded in correctly and fully seated. All I can say in addition is, it's not unusual for spark plugs to come loose in aluminum heads.
My first car was a Corvair. It used the metal crush gaskets on the plugs. I remember starting the engine with a remote starter and the hood open. It blew out a plug at startup and the wire kept it from hitting my uncle in the head.
I think that car wound up have six inserts installed for the plugs?
It can really depend on if your head uses a compressible gasket or a tapered seat design. The gaskets seem to be the inferior design and whatever you do NEVER try to install or remove spark plugs on an aluminum head unless the engine is COMPLETELY COLD.
Aluminum threads expand to the point that a hot engine permits the threads to strip out completely when the assembly is hot. It's my opinion that all aluminum heads should have steel inserts for the spark plugs permanently installed by the manufacturers. Then you could torque the plugs in place. They disagree with me.
Look at that plug. Make sure the head threads aren't stripped out and make sure there are no aluminum remnants in the plug threads. Aluminum heads are not fool proof. They have a lot of issues iron heads don't.